Threaded screws, bolts, nuts and the like are commonly used in industry to join elements of an apparatus to one another. The bolts are commonly tightened with tools such as spanners or wrenches that are old in the art. These spanners and wrenches generate a large angular moment as the bolt is being tightened in order to achieve a tight fit between the elements.
In some applications, however, the amount of force applied to the bolt must be strictly controlled. For example, when an engine cover is joined to an engine body using bolts, the torque on each of the bolts must be strictly controlled in order to distribute the pressure generated by the bolts equally over a gasket between the engine body and the engine cover. Using an ordinary spanner or wrench, equalizing the torque on each of the bolts depends upon the skill of the individual worker performing the tightening operation. This leads to inconsistent results and concomitant quality control problems resulting in a loss of sales and good will to the manufacturer.
One solution to the problem of uniformity in torque application to tighten bolts has been to use a tool known to the art as a torque wrench. The torque wrench typically has a ratchet gear mounted on a fitting which engages a bolt to be tightened. The ratchet gear is meshed with a pawl member urged against the ratchet gear by a spring. When the bolt is tightened, force is applied to the bolt until a level of torque equivalent to the spring force is attained. Upon exceeding the spring force, the ratchet gear and the pawl are disengaged, allowing the pawl to turn freely, preventing further torque from being applied to the bolt. Thus, an upper limit is set on the amount of torque applied to the bolt.
The major problem with a torque wrench of the above-described design is that the spring force is not easily determined when the tool is in use. While the torque limit can be adjusted to meet the needs of varied applications, the operator does not know how much torque has been applied or how much more is needed at any given time during the operation. Thus, there is a need in the industry for an adjustable tool wherein the amount of torque applied to a bolt can be varied with the application and wherein the tool displays the amount of torque being applied at any time during the tightening operation.